Printing blankets are generally made of one or more layers of cloth and other materials and generally have at least one surface coated with a rubber or other elastomeric material which acts as a transfer surface to take the ink image from the printing plate and transfer it to the paper on which it is to be printed.
Conventionally, these printing blankets are mounted onto the outer circumferential surface of a printing cylinder and are secured in place by various attachment means, such as reel rods or clamps. See for example, U.S. Pat. Nos. 4,261,262 and 3,675,573.
In order to ensure a secure attachment, the leading and trailing edges of the blankets are reinforced with a rigid bar, known in the industry as a blanket bar.
Blanket bars are generally made of metal, such as strip steel or extruded aluminum and are held in place by screws, bolts, rivets and/or adhesives.
One type of blanket bar has a "V" or "U" shaped channel with two extending arms which surround the edge of the blanket. For example, U.S. Pat. No. 3,675,573 shows a "V" shaped bar of extruded aluminum with each arm having an inwardly directed ridge so that when the arms are compressed into contact with the blanket edge, the ridges grip the blanket edge and hold the bar in place.
Another example is shown in U.S. Pat. No. 3,883,940 in which a series of prongs are formed in one arm of the bar. When crimped in place, the teeth bite into the blanket to fasten the bar to the blanket.
All of the bars discussed above suffer from serious drawbacks. For example, adhered bars are difficult to work with, labor intensive, cumbersome, expensive and frequently fail. When there is an adhesive failure, there is immediate separation of the blanket from its retaining device and the results are often catastrophic and can include damage to the printing press. Likewise, riveted bars and ridged bars, as discussed above, cause the blanket to pucker which increases the blanket thickness and tends to weaken the blanket in that area. Further, bars with teeth, as discussed above, are often subject to failure as these bars tend to open up, allowing the blanket to slip out of the bar and retaining means.
These problems have become even more severe in newer presses where the gap in the blanket cylinder has become narrower.
The newer presses have blanket cylinders where the gap, through which the ends of blanket are inserted and retained, has been narrowed from about 0.250 inches to less than 0.210 inches and more preferably less than 0.190 inches. Narrowing the gap reduces the incidence of cylinder "bounce" during printing. Bounce is a vibration caused by the impact of the edges of the gap against an adjacent printing roll. Bounce adversely affects (1) printing quality, causing streaks and runs and (2) machine wear, causing a rapid increase in the rate of wear of the bearings, bearers, cylinders, etc. and (3) may also reduce blanket life. The narrow gap also increases the number of printable lines on a cylinder of given size and reduces paper trim losses. Because of these benefits and the development of pinless folders among other factors, presses equipped with blanket cylinders having narrower gaps have become increasingly more popular. These newer presses are sometimes referred to as narrow gap presses.
Narrow gap presses require thinner blankets and thinner bars. Extruded aluminum bars cannot be made thin enough to be used on a narrow gap press and still provide the required rigidity and retention strength. Likewise, riveted bars, because they pucker the blanket and increase the blanket thickness, are not acceptable. The "U" shaped bar as disclosed in U.S. Pat. No. 3,883,940 is impractical in a thin design because there is not sufficient strength in the bar to prevent it from opening up and allowing the blanket to slip out of the bar and retaining means. The problems with adhered bars become magnified in narrow gap presses.
The present invention while preferably used on narrow gap presses, can be used on other presses as well, and provides a simple, efficient means for ensuring the retention of the bar to the blanket.